The ISS consists of four separate subsystems:
Data transmission from an ISS site can be via modem and phone line or via satellite. In the TOGA COARE project, ISS data from island ISS sites in the Pacific were transmitted to the GOES WEST satellite and ultimately relayed to Boulder, Colorado and other project locations. WMO format GTS messages, both sounding and surface, were transmitted in real time from the TOGA COARE ISS sites. These messages were transmitted via the GOES WEST satellite to forecast centers in real time and incorporated into the forecast models. (Those messages and others were also transmitted to Boulder and then relayed back to Townsville, Australia.)
The balloonborne radiosonde navaid (Loran or Omega) sounding system is the standard NCAR "CLASS" sounding system. This sounding system typically uses the Vaisala RS-80 L or the Vaisala RS-80 N radiosondes which use Loran and Omega radionavigation signals respectively for windfinding. Launching configurations can vary depending on the installation. An ISS site may have an enclosed air conditioned launcher, a "bag" launcher, or possibly no launcher at all. The balloon and sonde are secured to structures available before release in the latter case.
The enhanced surface observing station consists of two instrumented towers and a rain gauge. A ten-meter tower is instrumented with wind velocity sensors as well as pressure, temperature, and humidity sensors. A separate two-meter tower is typically instrumented with radiometers. The data are formatted and processed by a Campbell CR10 datalogger. The datalogger is programmable. It is typically configured to generate one-minute average data which are sent via RS-232 to the Sun workstation. Shipboard surface observing station installations require some type of shipboard navaid to correct for ship velocity in the wind velocity measurement.
The 915 MHz wind profiler deployed during TOGA COARE was a three-beam Doppler beam swinging system. The system is now being modified to a five-beam system. The profiler is a long-wavelength Doppler radar which detects the backscattered signals from turbulence-induced refractive index variations. The profiler thus tracks the motion of the turbulent eddies which drift with the mean flow, providing a measurement of the mean wind velocity.
Radial wind velocities are obtained over a variable number of range gates (e.g. 25 to 40) using spectral moment and consensus averaging techniques. Consensus averages can be calculated over varying periods for two vertical modes: one for lower altitude sampling with a high vertical resolution and one for higher altitude sampling with reduced vertical resolution. The raw data are recorded on site in real time to allow for alternative post-processing.
Any shipboard ISS profiler installation requires special attention and adaptation. The microstrip profiler antenna needs to be mounted on a gyrostabilized platform. A three axis accelerometer may be required to measure oscillatory ship motions (backscattered signals are doppler shifted by the motion of the ship) and a GPS receiver is used to determine the speed and heading of the ship. Details of shipboard testing of this system can be found in Carter, et al. (1992).
The radio acoustic sounding system (RASS) utilizes the vertical beam of the profiler to track a broad band acoustic frequency wave front produced by four speakers. (The radar actually tracks refractive index perturbations induced by the acoustic wave traveling at the local speed of sound.) The broad band frequency is used to assure that the Bragg criterion is met. A vertical virtual temperature profile is obtained from the tracking of this wave front (acoustic shell) from the relationship between air density and the propagation velocity (speed of sound) of that wave front.
Vertical virtual temperature profiles with 100 meter resolution can be obtained periodically by the tracking of the generated acoustic signals. A correction can be applied in the real time processing to remove the vertical wind motion from the measured speed of the acoustic shell. Note that the RASS temperature profiles obtained in real time will be drastically affected by hydrometeors. The return from the hydrometeors will invalidate the measurement of vertical wind and the resultant correction will give erroneous profiles. In some cases where there are hydrometeors present, it may be possible to obtain a reliable shell propagation speed in post-processing by using rainfall rate measurements to estimate and remove the hydrometeor fall speeds which would otherwise contaminate the vertical wind measurement.
The ISS sites are housed in a standard 20-foot sea container modified to serve as an equipment shelter and laboratory for project scientists and engineers. The modified sea container houses the Sun workstation, the profiler/RASS computer, the balloonborne sounding system computer, the profiler antenna array, as well as storage for expendables, disks, and tapes. The container may or may not have a balloon launcher attached. The RASS speakers are typically placed around the outside of the container. The surface meteorological instrumentation and Campbell datalogger are outside away from the container.
The radiation sensors are mounted on a one meter boom on the top of a separate two-meter tower. The standard ISS radiation sensors include an up-looking solar radiation sensor and a net radiation sensor. In situations which require more complete radiation measurements, additional sensors can be added.
The output from all the sensors is directed to the Campbell datalogger for processing. The Campbell datalogger, which is independently programmable, typically generates one-minute average data which are sent via RS-232 to the ISS Sun workstation. The data input to the Campbell datalogger are five-second sample data.
The LI-COR LI200S pyranometer utilizes a silicon photodiode which has a spectral response in the wavelength band from about 0.4 um to about 1.2 um. The LI200S is calibrated against an Eppley Precision Spectral Pyranometer (PSP). The calibration is periodically checked. The absolute error of the calibration is + / - 5.0% maximum with the typical error being + / - 3.0%. The cosine response has been corrected up to an 80 degree incidence angle.
The Radiation and Energy Balance Systems, Inc. Fritschen net radiometer, measures the sum of all incoming radiation (direct solar, diffuse solar, longwave skylight) minus the sum of all outgoing radiation (reflected radiation and terrestrial longwave radiation). A single data stream, the difference of incoming and outgoing radiation, is recorded from the Radiation and Energy Balance Systems, Inc. Fritschen net radiometer. The wavelength range of this instrument covers both the shortwave and longwave bands.
When more precise radiation measurements are required, three separate radiation measurements can be made in the surface meteorological installation, shortwave, longwave, and net radiation. Both the shortwave and longwave radiation measurements are then made with pairs of Eppley radiometers, one up-looking and one down-looking radiometer in each pair. The net radiation measurement is again made with the Radiation and Energy Balance Systems, Inc. Fritschen net radiometer.
The shortwave radiation measurements are made with Eppley PSP pyranometers. The wavelength range of these instruments is 0.3 to 3.0 um. Separate data streams are recorded from both the up-looking and down-looking pyranometers. The longwave radiation measurements are made with Eppley pyrgeometers. The wavelength range of these instruments is 4.0 to 50.0 um. As with the pyranometers, separate data streams are recorded from both the up-looking and down-looking pyrgeometers.
The following tables summarize the wind profiler and RASS system characteristics. Note that on the wind profiler height resolution entries, resolutions are given for two modes, a low altitude mode (high resolution) and a high altitude mode (low resolution). The height to which the profiler can measure wind velocities is highly dependent on the atmospheric characteristics.
TABLE 12 Wind Profiler Typical Performance Specifications ------------------------------------------ Radar Frequency 915 MHz Radar Wavelength 0.33 meters Mean Power 7.0 W Antenna size 4.0 square meters Beam Width 9.0 degrees Minimum Height 0.12 km AGL Maximum Height 2 to 5 km AGL Height Resolution
Low altitude mode:
60 & 100 meters
High altitude mode: 250 & 500 meters Wind Speed < 1.0 m/s Wind Direction < 1.0 degree ------------------------------------------
TABLE 13 RASS Specifications ----------------------------------- Acoustic Frequency 2000 Hz Acoustic Power:
40.0 W Total Electrical Input Acoustic Beamwidth 8.0 degrees -----------------------------------